Interblade platform for a fan, rotor of a fan and associated manufacturing method

ABSTRACT

The invention concerns an interblade platform ( 3 ) of a turbomachine fan, characterized in that it is produced: 
     in an aluminum alloy belonging to the 7xxx series that has undergone full-temper heat treatment of the T7x type, or 
     in an aluminum alloy belonging to the 2xxx series that has undergone heat treatment of the T6x or T8x type.

FIELD OF THE INVENTION

The invention concerns a turbomachine fan, for example an aircraft turbojet or turboprop. In particular, the invention concerns the manufacture of an interblade platform, in particular for a turbomachine.

TECHNOLOGICAL BACKGROUND

A turbomachine generally comprises, from upstream to downstream in the direction of flow of the gases, a fan, one or more compressor stages, for example a low-pressure compressor and a high-pressure compressor, a combustion chamber, one or more turbine stages, for example a high-pressure turbine and a low-pressure turbine, and a gas exhaust nozzle.

As is known, a turbomachine fan comprises a rotor disc carrying a plurality of blades, the roots of which are engaged and held in substantially axial grooves formed at the periphery of the disc. These blades are associated, at the radially innermost ends thereof, with platforms able to reconstitute the aerodynamic profile of the annular stream of flow of the air entering the turbomachine in order to ensure continuity of the flow stream and to optimize the air flow between the blades.

Each blade of the fan can be formed in a single piece with a platform. The blade is in this case relatively heavy, which results in high mechanical stresses at the blade root in use, which may cause the appearance of cracks or fissures on this root. In addition, the loss of a fan blade of this type causes significant damage in the turbomachine and considerably increases the imbalance in the fan.

The fan blades and platforms may also be independent of one another. The platforms, referred to as interblade platforms, are then each attached and fixed to the fan disc between two adjacent blades. In the event of the loss of a blade, the two platforms situated on either side of the blade then remain in place on the disc and do not cause any additional damage in the turbomachine. Moreover, they also protect the adjacent blades from the debris from the lost blade.

A space is provided between the platforms and the blades to allow the latter a limited movement during the various operating phases of the engine. This space does however disturb the aerodynamic profile of the flow stream, thus reducing the flow rate and efficiency of the fan. It has therefore been proposed to block this space with a seal, some of which comes into abutment against the adjacent blade of the fan. Generally, the seal is produced from an elastomer material with an elongate shape with a profile that is constant over its entire length. Nevertheless, after a certain period of operation, it has a tendency to wear and to break locally, thus reducing the seal between the platforms and the adjacent blades, so that it is necessary to replace them regularly during the life of the turbomachine.

It was therefore proposed, in the document FR 2 939 836, to replace the conventional seals with seals of elongate form, at least one of which has a transverse section in a developing, that is to say non-constant, shape between the two ends thereof. In particular, the cross-section of the part of the seal that is in contact with the blade that is adjacent to it is bulbous and increases from one end of the seal to the other. The choice of a seal shape according to the stresses to which it is subjected in operation so as to limit the maximum values of the stresses thus makes it possible to avoid deformations liable to exceed the rupture limit of the seal and to reduce its frequency of replacement.

Nevertheless, this seal with a developing shape must be secured to the platform by bonding with the platform. However, this bonding step requires polymerizing the elastomer material constituting the seal at a temperature above 150° C. for at least fifteen minutes (hot bonding) and degrades the corrosion resistance of the interblade platform. This is because the interblade platform, which is generally produced from 7449 T6 aluminum, undergoes a surface treatment comprising shot blasting and anodizing steps in order to improve its mechanical strength and corrosion resistance. However, the effect of the polymerization heat treatment is to cause a crazing of the protective layer obtained by anodization, thus creating cracks that degrade its corrosion resistance. In addition, the interblade platforms are rotating parts that undergo high mechanical stresses throughout the operation of the fan, which has a tendency to enlarge the cracks and consequently to reduce further their corrosion resistance.

It has been proposed to bond the seal against the platform without carrying out polymerization heat treatment (cold bonding), in order not to degrade the anodization layer. Nevertheless, these bondings do not have sufficient strength to hold the seal in place throughout the service life of the platform, so that the seal ends up by becoming detached.

It was therefore proposed to replace the seal with a developing shape with lighter seals or ones having a different transverse section, for example lip seals (the contact cross-section of which is this time not bulbous), in order to reinforce the strength of the cold bondings and to modify the stresses applied to the seal. However, these lighter seals do not guarantee sufficient sealing between the platform and the adjacent blade during all the use phases (that is to say from tickover to maximum speed) of the turbomachine, which causes a loss of aerodynamic performance of the turbomachine. In addition, in the case of the lip seal, the lip has a tendency to jam between the blade and the interblade platform and therefore risks tearing.

It is moreover not possible to fix the seal mechanically, for example by means of bolts or rivets or by means of attached pieces, because of the geometry of the platform, the small amount of space available between the platform and the adjacent blade, and the weakening of the platform due to the piercings and increase in weight caused by such fixings, which risk reducing the service life of the platform.

SUMMARY OF THE INVENTION

One objective of the invention is therefore to propose an interblade platform for a turbomachine fan that is capable of guaranteeing the seal between the adjacent blades while having high resistance to corrosion and sufficient mechanical strength despite the environment with high mechanical stresses.

For this purpose, the invention proposes a method for manufacturing an interblade platform that comprises a plate and two lateral ribs extending from a face of said plate, the method comprising the steps consisting of:

-   -   providing a piece made from an aluminum alloy belonging to one         of the series 2xxx or 7xxx, said piece having the shape of the         interblade platform,     -   carrying out heat treatment of the piece,     -   anodizing the interblade platform,     -   placing a seal against each lateral rib of the interblade         platform, and     -   bonding the seals to the interblade platform, the method being         characterized in that:         the heat treatment is of the type         -   T7x in the case of an aluminum alloy belonging to the 7xxx             series, or         -   T6x or T8x in the case of an aluminum alloy belonging to the             2xxx series,             and in that the interblade platform does not undergo a shot             blasting treatment prior to the anodization step.

In one embodiment, the interblade platform is produced from an aluminum alloy belonging to the 7xxx series and the full-temper heat treatment is of the T73 or T74 type, or of the T7351 type.

According to a second aspect, the invention also proposes an interblade platform of a turbomachine fan obtained according to a manufacturing method as described above, comprising a plate and two lateral ribs extending from a face of said plate, characterized in that it is produced either from an aluminum alloy belonging to the 7xxx series that has undergone full-temper heat treatment of the T7x type, or from an aluminum alloy belonging to the 2xxx series that has undergone heat treatment of the T6x or T8x type.

Some preferred but non-limitative features of the above platform are as follows:

-   -   the aluminum alloy is a grade 7x75 aluminum alloy,     -   the aluminum alloy comprises at least one of the following         alloys: 7075, 7175, 7475, 7010, 7040, 7050, 2016, 2050, 2219 or         2618, and     -   the aluminum alloy belongs to the 7xxx series that has undergone         full-temper heat treatment of the T73 type.

According to a last aspect, the invention proposes a rotor of a turbomachine fan characterized in that it comprises an interblade platform as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, aims and advantages will appear more clearly from a reading of the following detailed description, made with reference to the accompanying figures given by way of non-limitative examples and in which:

FIG. 1 is a view from below in cavalier perspective of an example embodiment of an interblade platform according to the invention,

FIG. 2 is a plan view in cavalier perspective of the interblade platform of FIG. 1,

FIG. 3 is a view in transverse section of the interblade platform of FIG. 1,

FIG. 4 is an exploded view in cavalier perspective of part of an example embodiment of a fan rotor of a turbomachine, in which the interblade platform of FIG. 1 has been shown, as well as an example of a blade that can be fixed to the rotor, and

FIG. 5 is a flow diagram showing various steps of an example embodiment of the method for manufacturing an interblade platform according to the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

A fan rotor of an engine comprises in particular a rotor disc 1 carrying a plurality of blades 2 engaged and held in corresponding grooves 12, oriented substantially axially with respect to an axis XX of the engine and formed in a rim 10 of the disc. These blades 2 are associated at their radially innermost ends with interblade platforms 3.

The blades 2 have a root 20 and a vane 22 and are inserted in the grooves 12 at their root 20. The grooves 12 therefore have a shape roughly complementary to the blade roots 20 that they receive, so as to prevent any play between the blade 2 and the disc 1 of the rotor.

The rotor comprises the same number of platforms 3 as blades 2, the platforms 3 being placed between two adjacent blades 2.

An example of an interblade platform 3 is shown in FIGS. 1 and 2. An interblade platform 3 comprises a plate 30 with a roughly frustoconical shape adapted to delimit the radially innermost part of the flow stream of the air flow 5 guided between two adjacent blades 1, as well as two lateral ribs 32 that extend radially towards the insert from a bottom surface of the plate 20, close to the lateral ends 31 of the plate 30. The interblade platform 3 is fixed to the disc 1 by means of three radial retaining tongues 34, 36, 38 placed in a row between the lateral ribs 32 along the platform 3. The upstream tongue 34 (in the direction of flow of the stream) is pierced axially and can be fixed by means of a retaining plate to an upstream flange 14 on an upstream face of the disc 1. The intermediate tongue 36, also radial, can be held by a key on a radial tongue 16 positioned on the rim 10 of the disc 1 between two groves 12. Finally, the downstream tongue 38 can be connected to the drum of the compressor immediately downstream by a radial tongue 18. The keys are oriented parallel to the flow of the stream 5 and enable the interblade platform 3 to be held both axially and radially on the disc 1.

In order to ensure the impermeability of the rotor, seals 4 made from elastomer material are arranged along the lateral ribs 32 of the plate 30, on either side of the interblade platform 3. They may be conventional seals or seals with a developing shape such as those described in the document FR 2 939 836.

An example of a seal 4 with a developing shape able to be used has been illustrated for example in FIG. 3. The seal 4 comprises an attachment part 40, a flexible connection part 42 and a contact part 44. The seal is secured to the platform 3 by adhesion of the attachment part 40 along the plate 30, between the corresponding lateral rib 32 and lateral end 31 of the plate 30, the contact part 44 remaining free. Optionally, a groove can be formed in an external surface of the lateral ribs 32 in order to receive the seal 4. In a variant, the lateral rib 32 has a local protrusion for positioning the seal 4 precisely on the interblade platform 3. The contact part 44 preferably has a bulbous, oval or circular cross-section in order to ensure good tangential contact with the adjoining part or to prevent the seal 4 becoming wedged between the adjacent blade 2 and the interblade platform 3, and confers stiffness on the seal 4. FIG. 3 illustrates for example the case of a contact part 44 rounded in shape. The connection part 42 for its part is narrower than the contact part 44 and is flexible to enable the seal 4 to adapt to the stresses that it receives. Moreover, the contact part 44 can extend beyond the lateral end 31 of the plate 30 in order to come into contact with the adjacent blade root 20.

In order to prevent the step of adhesion of the seal 4 to the interblade platform 3 degrading the corrosion resistance of the interblade platform 3, the platform 3 is produced from an aluminum alloy belonging to the 7xxx series that has undergone full-temper heat treatment. According to one embodiment, the step of shot blasting the interblade platform is also omitted. In this way, metallurgical states compatible with the adhesion heat treatment of the seals 4 is obtained, eliminating the risks of crazing of the anodization, and also having better corrosion resistance than the aluminum alloys normally used for the platforms 3 as well as good dimensional stability.

Moreover, the use of these aluminum alloys makes it possible to keep the existing elastomer seals 4 and to continue to bond them using the normal polymerization methods, thus guaranteeing a good seal for the rotor throughout the period of operation of the fan, without any risk of detachment of the seals 4.

The aluminum alloys in the 7xxx series comprise zinc as the main alloy element. Grades that can be used in particular are the 7075, 7175, 7475, 7010, 7040 and 7050 grades.

Full-temper heat treatment consists of performing, at the end of the putting in solution and quench hardening, an artificial tempering, generally two-stage, beyond the hardening peak of the material, thus desensitizing the material to corrosion. It is usually designated by T7x in the aluminum alloy denomination. Full-tempering may be carried out at several degrees, each degree being designated by a figure after T7, ranging from 9 (for low full-tempering) to 3 (for maximum full-tempering). Here, high full-tempering will for example be favored, in particular T73 or T74. The heat treatment may also be of the T7351 type (that is to say a full-temper of the T73 type followed by detensioning by traction after quenching).

In a variant, the interblade platform 3 may also be produced from an aluminum alloy in the 2xxx series that has undergone suitable heat treatment at a high temperature (around 150° C. to 200° C.), in particular of the T6x or T8x type. Aluminum alloys in the 2xxx series comprise copper as the main alloy element. For example, it may be an aluminum alloy in the 2016, 2050, 2219 or 2618 grades.

The use of such a heat treatment on an aluminum alloy in the 2xxx series obtains a dimensionally stable part having good corrosion resistance.

Such aluminum alloys are known from the aeronautical field. Nevertheless, it was in no way obvious for a person skilled in the art to use them for producing interblade platforms, in particular for a turbomachine fan, since these are rotating parts subjected to very strong mechanical stresses. These alloys have lower mechanical strength than conventional alloys of the 7xxx type that have undergone heat treatment of the T6 type, which would therefore have been a more obvious choice for a person skilled in the art.

The applicant however realized that the alloy in the 7xxx series that has undergone heat treatment of the T73 type was a very good compromise between mechanical strength and corrosion resistance, under the conditions of use of interblade platforms. It therefore constitutes a good example of an alloy that can be used for a platform 3 according to the invention.

In addition to enabling the interblade platform 3 to keep its corrosion resistance under stresses despite the polymerization treatment undergone to bond the seals to its lateral ribs 32, the aluminum alloys in series 7xxx and 2xxx are furthermore more usual than the alloys normally used for producing the platforms, in particular the 7449 aluminum alloy, which simplifies the procurement of raw material and reduces the costs of purchasing these raw materials.

According to the invention, the shot blasting step, which is normally carried out in the critical areas of the rotating parts made from aluminum alloy with a view to improving their fatigue strength, is omitted. This is because the applicant unexpectedly perceived that this step risked cracking the anodization. However, the particular choice of alloys in the 7xxx series that have undergone a heat treatment of the T7x type, or aluminum alloys belonging to the 2xxx series that have undergone a heat treatment of the T6x or T8x type, makes it possible to obtain an interblade platform 3 benefiting from acceptable mechanical strength, even in the domain of the rotating fan parts, without for all that degrading its corrosion resistance.

The use of these heat-treated aluminum alloys therefore has the advantage of alleviating the surface treatment of the interblade platform since they eliminate the necessity to perform a shot blasting step prior to the step of anodization of the platform, thus reducing the costs of manufacturing the interblade platform, as well as their manufacturing time.

The interblade platform 3 can be obtained from a metal part produced from the required aluminum alloy in the 2xxx or 7xxx series, which is finished 120, for example by forging, in order to obtain the required form for the interblade platform 3, before being heat treated 130 by T7 full-tempering or artificial annealing of the T6x or T8x type according to the series of aluminum chosen (7xxx or 2xxx respectively) for the interblade platform 3.

For example, the starting metal part may be a bar the top surface of which has the form of the flow stream and comprising the three radial holding tongues 34, 36, 38 previously produced by swaging.

The interblade platform 3 then undergoes conventional anodization treatment 140, without prior shot blasting, and then the seals 4 are arranged 150 between the plate 30 and the lateral ribs 32 in an area adjacent to the lateral ends 31 of the plate 30.

Finally, the seals 4 are bonded 160 to the interblade platform 3 by a conventional polymerization treatment.

The interblade platform 3 then has very good resistance to corrosion, in particular to stress corrosion, and good dimensional stability, while providing a good seal with the adjacent blades 2 of the rotor throughout the operating life of the fan, without requiring regular replacement of the seals 4 (in particular in the case where the seal corresponds to the one described in the application FR 2 939 836). 

1. Method for manufacturing an interblade platform that comprises a plate and two lateral ribs extending from a face of said plate, the method comprising the steps consisting of: providing a piece made from an aluminum alloy belonging to one of the series 2xxx or 7xxx, said piece having the shape of the interblade platform, carrying out heat treatment of the piece, anodizing the interblade platform, placing a seal against each lateral rib of the interblade platform, and bonding the seals to the interblade platform, wherein: the heat treatment is of the type T7x in the case of an aluminum alloy belonging to the 7xxx series, or T6x or T8x in the case of an aluminum alloy belonging to the 2xxx series, the interblade platform does not undergo a shot blasting treatment prior to the anodization step.
 2. Manufacturing method according to claim 1, wherein the interblade platform is produced from an aluminum alloy belonging to the 7xxx series, and the full-temper heat treatment is of the T73 type.
 3. Manufacturing method according to claim 2, wherein the full-temper heat treatment is of the T7351 type.
 4. Manufacturing method according to claim 1, wherein the interblade platform is produced from an aluminum alloy belonging to the 7xxx series, and the full-temper heat treatment is of the T74 type.
 5. Interblade platform of a turbomachine fan, comprising a plate and two lateral ribs extending from one face of said plate, characterized in that it is obtained according to a manufacturing method according to claim 1 and in that it is: in an aluminum alloy belonging to the 7xxx series that has undergone full-temper heat treatment of the T7x type, or in an aluminum alloy belonging to the 2xxx series that has undergone heat treatment of the T6x or T8x type.
 6. Interblade platform according to claim 5, wherein the aluminum alloy is a grade 7×75 aluminum alloy.
 7. Interblade platform according to claim 5, wherein the aluminum alloy comprises at least one of the following alloys: 7075, 7175, 7475, 7010, 7040, 7050, 2016, 2050, 2219 or
 2618. 8. Interblade platform according to claim 5, wherein the aluminum alloy belongs to the 7xxx series that has undergone a full-temper treatment of the T73 type.
 9. Rotor of a turbomachine fan, characterized in that it comprises an interblade platform according to claim
 5. 